Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
  • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
  • G01J3/12—Generating the spectrum; Monochromators
  • G01J3/26—Generating the spectrum; Monochromators using multiple reflection, e.g. Fabry-Perot interferometer, variable interference filters
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B26/00—Optical devices or arrangements for the control of light using movable or deformable optical elements
  • G02B26/001—Optical devices or arrangements for the control of light using movable or deformable optical elements based on interference in an adjustable optical cavity
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B26/00—Optical devices or arrangements for the control of light using movable or deformable optical elements
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B5/00—Optical elements other than lenses
  • G02B5/20—Filters
  • G02B5/28—Interference filters
  • G02B5/284—Interference filters of etalon type comprising a resonant cavity other than a thin solid film, e.g. gas, air, solid plates

Definitions

• the invention relates to a method for producing an electrically tunable Fabry- Perot interferometer, an electrically tunable Fabry-Perot interferometer, an inter- mediate product, and an electrode arrangement. More specifically, the invention relates to electrically tunable Fabry-Perot interferometers which are produced with micromechanical (MEMS) technology.
• MEMS micromechanical
• Fabry-Perot interferometers are used as optical filters and in spectroscopic sensors, for example.
• the Fabry-Perot interferometer is based on parallel mirrors, whereby a Fabry-Perot cavity is formed into a gap between the mirrors.
• the pass band wavelength of a Fabry-Perot interferometer can be controlled by adjusting the distance between the mirrors i.e. the width of the gap.
• the tuning is usually made electrically. It is common to use micromechanical technology for producing electrically tunable Fabry-Perot interferometers. Such a solution is described e.g. in patent document FI95838.
• Prior art structure of a micromechanical interferometer usually includes layers of silicon, wherein electrically conductive layers and reflective layers are doped.
• a movable mirror is provided by removing a sacrificial layer of silicon dioxide, which layer has initially been formed between two mirror layers. The position of a movable mirror is controlled by applying voltage to electrodes, which are included in the mirror structures.
• micromechanical production technology allows series production of interferometers.
• the doping of the silicon layers requires the use of an ion doping facility, which is expensive equipment and thus increases the production cost.
• Another problem relates to removing the sacrificial layer between the mirrors. In prior art processes the removal is a separate process which must be made before the interferometers can be cut from the wafers, and encapsulated. Such a separate process increases the complicity of the production process. Also, the cutting, encapsulating and transportation of the interferometers require special handling because of the movable, released mirror. A released mirror is sensitive to environmental stress, such as physical pressure, changes of temperature or humidity, contamination, etc.
• a further disadvantage of the prior art technology relates to the inability to provide a gap with short distance between the mirrors. This is due to the wet etching process wherein providing narrow gaps is difficult. Also, when Fabry-Perot interferometers are produced for visible and ultraviolet light, the optical layers need to be thin. Thin membranes are often discontinuous and include pinholes. Such membranes easily become damaged during wet etching. Therefore the prior art technology is not suitable for producing electrically tunable Fabry-Perot interfer- ometers for short wavelengths such as visible and ultraviolet range.
• the purpose of the present invention is to avoid or reduce disadvantages of the prior art.
• the object of the invention is achieved with a solution, in which polymer is used for providing a sacrificial layer between the mirrors of an electrically tunable Fabry-Perot interferometer.
• a mirror layer which is deposited after providing the sacrificial layer is usefully made in a process wherein the temperature of the sacrificial layer does not ex- ceed the glass transition temperature of the polymer material.
• the glass transition temperature of a polymer material is a temperature value, above which the structure of the polymer material becomes unstable and deformations of the structure generally occur.
• ALD atomic layer deposition
• electrodes of the mirrors are formed by using sputtering or evaporation of suitable metal, such as aluminium, copper, gold or platinum.
• first mirror structure is provided on the substrate
• second, movable mirror structure is provided, whereby the first and second mirror structures comprise first and second mirrors which are substantially parallel
• a Fabry-Perot cavity is provided between the first and second mirrors, whereby providing the cavity comprises providing a sacrificial layer between the first and second mirror structures before providing the second mirror structure, and the sacrificial layer is removed after providing the second mirror structure,
• first and second mirror struc- tures comprise first and second mirrors which are substantially parallel
• the cavity has been formed by providing a sacrificial layer on the first mirror structure, and at least partially removing the sacrificial layer after providing the second mirror structure, - electrodes for electrical control of the distance between the mirrors, is characterised in that the cavity has been made with sacrificial layer comprising polymer material.
• An intermediate product of an electrically tunable Fabry-Perot interferometer ac- cording to the invention comprising
• first and second mirror structures comprise first and second mirrors which are substantially parallel, - a sacrificial layer between the first and second mirror structures,
• a micromechanical component according to the invention which component has a first part and a second part, in which component there is a first electrode in the first part and a second electrode in the second part, and wherein the distance between the first part and the second part is controlled by applying a voltage be- tween the first and second electrodes, said first and second electrodes forming a first capacitance, is characterised in that there is a third electrode in the first part, wherein there is a second capacitance between the second and third electrodes, whereby the first and second capacitances are series connected, and the AC voltage applied between the first and third electrodes is adapted to form a control voltage across the first capacitance for controlling the distance between the first and second parts.
• the sacrificial layer can be removed with dry etching, and this can be performed after cutting the chips and also after encapsulating the chips. This allows simple cutting and packaging procedures because the movable mirror does not need to be released at that phase and is therefore not sensitive to environmental stress, such as physical pressure, changes of temperature or humidity, contamination, etc. Also, it is possible to transport the interferometers in normal transportation manners because the movable mirrors can be released after the transport.
• the interferometers according to the invention can be applied in new consumer applications where production quantities are large and production costs need to be minimal.
• the second, movable mirror structure comprises at least one layer which has been deposited in an environment wherein the temperature of the sacrificial layer remains below the glass transition temperature of the polymer material.
• the electrodes of the movable mirror structure are covered by electrically isolating or semi-insulating optical layer(s), which protects the electrodes and prevents electrical short circuits of the opposite electrodes.
• electrically semi-insulating layer such as TiO 2
• Charging of the optical layer may cause inaccuracy of the controlled mirror position.
• one of the electrodes is electrically floating. This allows the use of alternating voltage for the tuning of the interferometer, and it is not necessary to provide control conductors to one of the mirrors for controlling the gap between the mirrors.
• the production process of the interferometer is less complicated, and the structure is more reliable when it is not necessary to arrange electrical connections to all electrode layers.
• This floating electrode structure can be advantageous also in electrically tunable Fabry-Perot interferometers where other than polymer material is used as a sacrificial layer.
• the floating electrode structure can also be used in other components than interferometers, such as adjustable capacitors.
• the electrically floating electrode can be an electrode of a movable part or an electrode of a fixed part in the structure of a component. When the electrically floating electrode is located in the upper mirror of an interferome- ter, the number of lithography phases after applying the polymer layer can be minimized.
• the mirror structures have further electrodes for capacitive measurement of the width of the gap between the mirrors. Such measurement information can be used as a feedback in controlling the interfer- ometer.
• the interferometer is integrated with a radiation detector, whereby the detector can be formed into a silicon substrate of the interferometer.
• a radiation detector whereby the detector can be formed into a silicon substrate of the interferometer.
• UV ultraviolet
• NIR near infrared radiation
• IR infrared radiation
• mirror means a structure where there is a reflective layer.
• gap width means the distance between the mirrors at the concerned position.
• electrically floating means that the concerned electrode is not electrically connected to any functional potential of the interferometer including ground potential.
• sacrificial layer means a material layer which is at least partially removed in the final product.
• resistivity means resistivity which has a value within the range 1 * 10 "2 ...1 * 10 6 ohm metres.
• Figure 1 a illustrates a flow diagram of an exemplary process according to the invention for producing an electrically tunable Fabry-Perot interferometer
• Figure 1 b illustrates cross sections of an exemplary electrically tunable Fabry- Perot interferometer according to the invention after completing exemplary production process phases of Figure 1 a;
• Figure 2 illustrates a cross section of an exemplary monolithic integrated spectrometer according to the invention
• Figure 3 illustrates a top view of an exemplary electrically tunable Fabry- Perot interferometer according to the invention
• Figure 4a illustrates an exemplary pattern of an electrode
• Figure 4b illustrates an exemplary pattern of a counter electrode
• Figure 5a illustrates an exemplary electrode structure where the electrode of the movable mirror is electrically floating
• Figure 5b illustrates the equivalent circuit of the electrode structure of Figure 5a.
• Figure 1 a illustrates a process diagram of an exemplary method according to the invention for producing an electrically tunable Fabry-Perot interferometer.
• Figure 1 b illustrates cross sections of the product after certain production phases of Figure 1 a.
• the production process is started by providing a wafer 100 in phase 11.
• the wafer material can be e.g. fused silica.
• layers 101 -105, 108 are formed for providing layers of the first, fixed mirror structure on the substrate.
• the first mirror structure can be produced by e.g. depositing successive layers of titanium dioxide TiO 2 and aluminium oxide AI 2 O 3 on the substrate, phase 12.
• the thickness of the titanium dioxide layers can be e.g. 10 nm - 2 ⁇ m
• the thickness of the aluminium oxide layers can be e.g. 10 nm - 2 ⁇ m.
• the actual thickness of the layers depends on the materials and the range of wavelengths at which the interferometer needs to be functional.
• the thickness of the layers is typically a quarter or a half of the operating wavelength of the radiation within the material of the concerned layer.
• These layers can be deposited on the substrate by ALD process, for example.
• the temperature of the ALD process can be e.g. 100-300 0 C.
• the sacrificial layer of polymer has not yet been provided at this stage, it is also possible to use alternative processes which utilize higher temperatures.
• a layer of aluminium, 106 is sputtered on the TiO 2 layer 105.
• the thickness of the Al layer is e.g. 10-100 nm.
• This layer of aluminium 106 will provide a first mask layer for electrode and electric wiring.
• the sputtered aluminium layer is patterned, and wet etched in order to remove the layer from the required locations outside the pattern. This way, electrode 106 of a required pattern is formed.
• An optical layer of TiO 2 , 108 is then deposited above the Al layer by e.g. using ALD process.
• the thickness of this TiO 2 layer is e.g. 10 nm - 2 ⁇ m.
• the thickness of the second Al layer is e.g. 100 nm - 2 ⁇ m.
• the aluminium layer is patterned and etched to remove the aluminium from required locations outside the pattern, i.e. outside the electrode contact areas.
• the topmost TiO 2 layer at the optical area of the first mirror is thus made of two optical layers 105, 108, and the electrode 106 is located between these two lay- ers. This way a protective layer is provided on the electrode.
• TiO 2 layer is electrically slightly conducting, which prevents occurrence of charging phenomena at the surfaces of the electrodes. However, the electrical isolation of the TiO 2 layer is sufficient for providing the required isolation in the lateral direction.
• a layer of sacrificial polymer, 112 is provided by spinning process, for example. This layer is also patterned as a mask layer, and the polymer is removed outside the pattern.
• the sacrificial layer will define the Fabry-Perot cavity.
• the sacrificial layer is polymer material, and thickness of the sacrificial layer is defined by the required distance between the mirrors of the interferometer.
• phase 15 an optical layer of TiO 2 , 113, is deposited on the sacrificial layer by using e.g. ALD process.
• This mask layer of TiO 2 is etched and patterned, whereby the TiO 2 layer is removed from the required locations 114 outside the pattern, at the area of the electrode contacts 110.
• This layer serves as an optical layer of the second mirror and as protecting layer for the electrode.
• This layer is preferably electrically semi-insulating, whereby it prevents the appearance of charging phenomena in the second mirror of the finished structure. It also prevents short-circuit of electrodes.
• the layer is uniform, and therefore it is possible to remove the resist used for patterning electrodes without damaging the sacrificial layer of polymer material.
• a layer of aluminium 115 is sputtered to provide the electrode. The thickness of this aluminium layer is e.g. 10 nm - 100 nm.
• the aluminium mask layer forms the electrode 115 of the upper, movable mirror of the interferometer.
• the aluminium layer is patterned and wet etched in order to remove the aluminium from required locations 1 16 outside the pattern.
• phase 17 further optical layers are formed for the second mirror.
• the first and third layer is made of AI 2 O 3
• the second and fourth layer is made of TiO 2 .
• These optical layers can be deposited by using ALD process.
• the above ALD phases can be processed in a temperature of 100-300 0 C, for example. Other alternative deposition processes may also be used but the temperature of the deposition process must be low enough to avoid deformation of the sacrificial layer of polymer material.
• the thickness of the TiO 2 layer can be e.g. 10 nm - 2 ⁇ m
• the thickness of the AI 2 O 3 layer can be e.g. 10 nm - 2 ⁇ m.
• the actual thickness of the layers depends on the materials and the range of wavelengths at which the interferometer needs to be functional.
• the AI 2 O 3 and TiO 2 layers produced in phases 15 and 17 form a mask layer which is patterned and etched in phase 18. By the etching, the optical layers are removed from above the electric contacts of the electrodes, 121.
• the patterning and etching can also be used for providing through-holes in the second mirror. These holes are needed for removing the sacrificial layer 112.
• phase 19 an antireflection layer of e.g. MgF 2 , 122, is deposited on the surface of the silica wafer, which surface is opposite to the previously mentioned interferometer layers.
• the chips are cut, and the sacrificial polymer is dry etched with O 2 plasma.
• the upper, second mirror structure has small-sized holes in order to allow the reactive species to penetrate through the mirror structure.
• the Fabry-Perot cavity 123 is formed.
• the Figure of phase 20 thus shows a completed interferometer chip. The interferometer chips are then encapsulated and the electrode wires may be bonded.
• the removal of the sacrificing layer takes place after the encapsulation.
• the sacrificial layer can be removed: before the chip is cut out from the wafer; after the chip is cut from the wafer but before encapsulation of the chip; or after the encapsulation of the chip.
• the sacrificial layer is not re- moved before cutting the chips from a wafer, it is possible to use normal cutting procedures since the second mirror is not sensitive to environmental stress, such as physical pressure, changes of temperature or humidity, contamination, etc.
• Figure 2 illustrates a cross section of an exemplary monolithic integrated spectrometer according to the invention.
• the component has similar layers 101 -122 as was described in connection with Figure 1 b.
• the component has a functionality of a Fabry-Perot interferometer.
• the reflecting layers of the mirrors are provided by layers 103 and 118.
• the electrode 115 of the movable mirror is electrically connected to one of the electrical connections 110.
• the electrodes 106b of the lower, fixed mirror are electrically connected to another of the connections 110.
• the cavity of the interferometer is formed by the space 123, from which sacrificial polymer layer has been removed.
• the sacrificial layer is dry etched e.g.
• the polymer layer has been removed from the optical area of the interferometer wherein there are holes in the movable mirror. However, the polymer layer is not removed from the edges 112 of the polymer layer. The remaining polymer layer between the edges of the movable upper mirror and the lower fixed mirror serves as a support for the movable upper mirror. The polymer keeps the movable mirror in a straight and uniform position, and a suitable tension can be created and maintained for the movable mirror.
• the polymer layer also as a support for the movable layer, but it is also a possible alternative to provide the support for the movable mirror by applying a supporting layer above and over the edges of the movable mirror.
• a support can be made of aluminium, for example.
• the component of Figure 2 also has a semiconductor detector of radiation, and the component thus gives an electric signal output which is a function of radiation intensity which is penetrated through the Fabry-Perot interferometer.
• the substrate is made of silicon in this embodiment, and the detector has been formed by applying a p + doped area 231 , n " doped area 230, and n + doped area 232 in the silicon substrate as shown in Figure 2.
• a detector of an integrated spectrometer may be formed as a pn-junction, PIN-detector or a charge-coupled device (CCD), for example.
• the pn-junction and PIN detector can be made of silicon, germanium, or indium gallium arsenide, for example.
• Figure 3 illustrates a top view of an exemplary electrically tunable Fabry-Perot interferometer 30 according to the invention.
• the contacts 310a and 310b for the electrodes of the upper and lower mirrors are located at corners of the interferometer.
• the optical area 316 is circular, and the upper, second mirror is provided with plurality of small-size holes.
• the holes are used for removing the sacrificial layer by etching with oxygen plasma.
• the holes are preferably evenly distributed across the optical area of the second mirror.
• the diameter of each hole may be e.g. 100 nm - 5 ⁇ m.
• the holes may cover an area of 0,01 % - 5 % of the optical area of the second mirror.
• Such holes function mainly as reflecting mirrors and do not therefore have substantial on the performance of the interferometer.
• Figure 4a illustrates an exemplary pattern of electrodes of a first part
• Figure 4b illustrates an exemplary pattern of electrodes of a counter part
• the electrodes of Figure 4a can be e.g. electrodes of a fixed first mirror
• the electrodes of Figure 4b can be e.g. the electrodes of the second, movable mirror, or vice versa.
• the distance between the mirrors is controlled by applying a voltage between electrodes 406 and 415, which have electrical contacts 410a and 410b respectively.
• the electrode 435 determines the potential of the center part inside the electrode 435.
• Figure 5a illustrates an exemplary structure of electrodes where the electrode of of one of the mirrors is electrically floating.
• Figure 5b illustrates the equivalent circuit of the structure of Figure 5a.
• the floating electrode is controlled with alternating (AC) control signal.
• the control voltage is applied between contacts 510a and 510b.
• the control signal is lead from the contact 510b via the conductor 550 and a fixed capacitance Cfi ⁇ ⁇ d, 53 to the electrode 515 of the upper mirror 52.
• the second contact 510a is lead to the electrode 506 of the lower mirror 51.
• the circuit therefore includes two capacitances connected in series: fixed capacitance Cfi ⁇ ⁇ d , 53, and control capaci- tance C ⁇ ntroi, 54.
• the voltage between the electrodes 506 and 515 determines the distance between the mirrors 51 and 52 . This voltage can be controlled by controlling the amplitude V ac of the AC signal applied between the contacts 510a and 510b.
• the electrode of the upper, movable mirror is floating. It is also possible to provide an arrangement where an electrode of the lower, fixed mirror is floating. In this case the control voltage is connected to two electrodes of the upper, movable mirror.
• An electrode structure where one of the electrodes is electrically floating can also be applied in other types of micromechanical components. For example, it can be applied in electrically tunable Fabry-Perot interferometers which have been produced with other technology where, for example, other material than polymer is used as a sacrificial layer.
• the floating electrode structure can also be applied in other components than interferometers, such as in adjustable micromechanical capacitors.
• aluminium was described as a conductive material forming elec- trades, electrical wiring and connections.
• conductive materials such as copper, gold or platinum.
• silica and silicon has been mentioned as exemplary preferable materials for the substrate.
• AI 2 O 3 and TiO 2 layers were used as optical layers of the mirrors.
• tantalum oxide or silicon nitride can be used instead Of TiO 2
• silicon oxide can be used instead Of AI 2 O 3 .
• the materials of the successive layers have sufficiently high difference in their values of refractive index.
• optical layers of the mirror structures are produced by atomic layer deposition, and electrodes of the mirror structures are formed by sputtering or evaporation.
• other alternative processes are used. For example, it is also possible that sputtering or chemical vapour deposi- tion, CVD, is used for producing the optical layers.
• the inventive interferometers have several preferable applications. They can be used as controllable filters in optical spectrometers, colour analyzers, imagers, optical data communications, and in various devices for measuring e.g. contents of specific gases or liquids.

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• 2009
  • 2009-01-27 FI FI20095068A patent/FI125817B/fi active IP Right Grant
• 2010
  • 2010-01-27 US US13/146,319 patent/US20110279824A1/en not_active Abandoned
  • 2010-01-27 EP EP10735510.9A patent/EP2394146B1/en active Active
  • 2010-01-27 WO PCT/FI2010/050043 patent/WO2010086502A1/en active Application Filing
• 2019
  • 2019-07-29 US US16/525,213 patent/US11194152B2/en active Active

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